Connection cables with standardized round plug connectors, e. g. straight or 90° angled M12, are frequently used in automation systems to connect sensors or actuators to control units or power supply units. The sensors or actuators are supplied via the connection cables, and analog switch signals and/or data are simultaneously transmitted. A screw element is used for securely connecting a round plug connector (male/female) to a matching counterpart piece, said screw element being either configured as a union nut (female) or as a shield sleeve (male). Undesired electromagnetic interferences like inductive proximity switches may lead to malfunctions in the system. Sensors may, for example, output false switching signals.
The interferences may be inductive, or capacitive, or galvanic interferences. In particular, significant interferences can occur in the proximity of powerful electric motors. In order to prevent the impairment of the individual components of a system due to electromagnetic interferences, shielded connection cables are frequently used which are significantly more expensive than unshielded cables.
The shielded connection cables normally have cable shields made of a non-magnetic material, like copper or aluminum. The cable shields frequently consist of braided individual shields which are composed of sets of wires that are interwoven in opposite directions. The density and thickness of the wire mesh essentially define the quality of the shielding.
The shielding is frequently only connected to ground (reference potential) at one end of the cable. In order to suppress magnetic interferences the shielding has to be connected to both ends of the cable. In such cases, however, there is the risk of a ground loop. Ground loops occur, in particular, when two points of the grounding are connected to one another via different electrically conductive paths. The ground connection, in this case, is a closed loop. In certain circumstances ground loops can cause very high currents to develop on the shielding which contribute to interferences in the signal lines. Two grounding concepts have become established in the industrial sector. In the tree structure, all ground connections are routed separately to a defined collection point. In the mesh structure, there are several grounding points which are connected to one another. The individual shielding concepts have been optimized for specific interferences.
In order to realize different shielding concepts, different cable versions are offered by the providers of shielded connection cables. In a first version, the shield is not connected. In a second cable version, the shield is connected to both sides, i.e. the respective union nut or shield sleeve of the round plug connector is connected to the reference potential. In a third version, the shield is connected, and additionally connected to pin 5 of the round plug connector.
When planning a system, the user has to decide on a specific shielding concept and then wire the individual components of the system to one another accordingly. Only on starting up a system, will it become apparent if the selected shielding concept will actually have the desired success. If the functionality of the system is still impaired by electromagnetic interferences, the wiring will have to be replaced in an expensive and time-consuming manner, or other cost-intensive measures will have to be taken in order to eliminate the interferences.